The present invention relates to a pilot signal transmission and reception method and a base station and a mobile terminal of a data communication system with a variable data rate in which the packet data transmission rate is increased and the transmission delay is reduced.
WO99/23844 discloses a method and an apparatus for increasing the packet data transmission rate and reducing the transmission delay in a data communication system with a variable data rate.
Consider the pilot signal in the prior art. The pilot signal is used for various purposes. The first purpose is to detect a base station which controls a mobile terminal. The second purpose is for a mobile terminal to receive a pilot signal from at least one base station, estimate SINR (signal-to-interference plus noise ratio) which is a reception level ratio between the pilot signal of a desired base station and the pilot signals of the other base stations, and calculate the data transmission rate allowable in a downlink by the SINR value. The third purpose is the utilization for synchronous detection of the received data signal.
The second purpose which is related to this invention is explained. A mobile terminal receives pilot signals from a plurality of base stations. A pilot signal received from a base station associated with the mobile terminal is referred to as a desired pilot signal, and the level ratio (SINR) of the desired pilot signal with other received pilot signals is estimated. Based on this SINR value, the mobile terminal determines a data rate at the mobile terminal from the associated base station and sends a request for this data rate to the associated base station. The larger the SINR value, the higher the data rate which the mobile terminal requests from the base station. The base station transmits a downlink signal including the pilot signal to the mobile terminal in accordance with the data rate requested. In this process, if the mobile terminal estimates the SINR lower than the value commensurate with the actual condition, then, the data rate requested from the base station will be lower than it should be and the data transfer rate is reduced. In the case where the mobile terminal estimates the SINR higher than the value commensurate with the actual condition, on the contrary, the condition of the propagation path is worse than estimated, and therefore the reception at the mobile terminal fails with the result that the signal needs to be repeatedly retransmitted. In order to operate the data communication system with high efficiency, the mobile terminal is required to estimate the SINR value with high accuracy. In order for the mobile terminal to estimate the SINR value with high accuracy, the factors causing an interference are desirably reduced as much as possible. This is because each factor of interference would cause an estimation error of the interference power of a mobile terminal, and in the case where the interference factors are great in number, the estimation errors are accumulated.
Pilot signals are categorized into a common pilot signal transmitted to all the cell areas or all the sector areas into which a given cell is spatially divided, and a dedicated pilot signal transmitted only to a specified mobile terminal. The common pilot signal is generally used for the first purpose described above, while the dedicated pilot signal is generally employed for the second and third purposes.
As many dedicated pilot signals as mobile terminals exist. A base station, therefore, transmits a multiplicity of dedicated pilot signals at a time. An explanation is given below about a multiplexing scheme of a dedicated channel including the dedicated pilot signal and the dedicated data signal.
First, the conventional technique for multiplexing the channels in space domain is explained. A base station realizes the space division multiplexing by using different directivity gains between the dedicated channels for transmission to a given mobile terminal 1 and the dedicated channel for transmission to another mobile terminal 2. With the approach of the directions of the mobile terminals as viewed from the base station, however, the dedicated channel transmitted to the mobile terminal 1 is received as an interference by the mobile terminal 2 due to the effect of the directivity gain. An increased interference deteriorates the SINR estimation accuracy and the SINR of the mobile terminal 2. As the transmission rate to each mobile terminal is determined in accordance with SINR, the signal transmission rate from the base station to the mobile terminal 2 is therefore reduced, which in turn reduces the throughput of the transmission to each mobile terminal from the base stations as a whole.
Apart from the space domain, the code division multiplexing scheme can reduce the interference using the orthogonal code such as the Walsh code for the dedicated channels transmitted. According to this scheme, no interference occurs within a cell in the absence of a multipath. Unlike in the space division multiplexing, therefore, the interference is not changed according to the direction of the mobile terminal. In the case where there is a multipath or an interference occurs from other cells, however, all the pilot signals multiplexed act as an interference wave, thereby deteriorating the SINR estimation accuracy and the SINR of the mobile terminal.
Further, the time division multiplexing has been conceived in which dedicated channels are transmitted not at a time but sequentially according to a schedule. In this method, the multipath which may exist has no effect on the interference, and the fact that only one dedicated channel is transmitted in a given time domain is considered to reduce the effect of the interference from other cells. Since the pilot signals are transmitted intermittently, however, a longer time is necessary to estimate SINR than in other methods. In a mobile terminal requiring an action against the fast fading, therefore, the delay has a significant effect. As a result, even in the case where a high-accuracy SINR estimation is possible at the time point of measurement, the longer updating period poses the problem that the estimation result is different from the fast-varing propagation at the time of actual transmission from the base station to the mobile terminal. Consequently, the SINR estimation accuracy at the mobile terminal is deteriorated.